Bottom Line:
The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium.This is similar to the interval between two successive bursts of action potentials.We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

ABSTRACTWe have applied the perforated patch whole-cell technique to beta cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K(+) conductance. The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

Figure 4: Pharmacological characterization of Kslow. (A) Failure of tolbutamide (100 μM) to affect current amplitude. Note that the current response observed when stepping from −70 to −40 mV is reduced by tolbutamide reflecting closure of KATP channels (shaded areas). The horizontal lines indicate the peak amplitude and steady state current, respectively. (B) Effects of TEA (20 mM). The horizontal dotted lines indicate (from top to bottom) the peak current amplitude, under control conditions and in the presence of TEA, and the steady state current, respectively. (C) Concentration dependence of inhibitory action of TEA. The Kslow conductance (G) is expressed as the fractional current using the current amplitude in TEA-free solution as unity (Gcontrol). Note that inhibition is half-maximal at 5 mM and that 30% of the current is resistant to TEA. (D) Electrical activity evoked by 15 mM glucose in the same cell before and after addition of TEA.

Mentions:
Tolbutamide, an inhibitor of KATP channels (Ashcroft and Rorsman 1989), had no effect on the current (n = 3; Fig. 4 A), but reduced the current step obtained when stepping the membrane potential from −70 to −40 mV. The latter observation indicates that some KATP channels remained active in 5 mM glucose. The Kslow current was likewise unaffected by both charybdotoxin (100 nM) and apamin (1 μM), blockers of large- and small-conductance Ca2+-activated K+ channels, respectively (not shown). By contrast, the broad spectrum K+ channel blocker tetraethylammonium (TEA)1 reduced the amplitude of the Kslow current in a concentration-dependent manner. Fig. 4 B shows an example where 20 mM TEA reduced the slowly deactivating current by ≈70%. The concentration dependence of the inhibition is summarized in Fig. 4 C. Inhibition was half-maximal at ≈5 mM TEA and ≈30% of the current was resistant to TEA. The effect of TEA on the amplitude of the current evoked by the train was associated with dramatic changes of the action potential firing pattern and the bursts of action potential were replaced by large overshooting action potentials that were either generated singly or as groups of 5–10 spikes (Fig. 4 D).

Figure 4: Pharmacological characterization of Kslow. (A) Failure of tolbutamide (100 μM) to affect current amplitude. Note that the current response observed when stepping from −70 to −40 mV is reduced by tolbutamide reflecting closure of KATP channels (shaded areas). The horizontal lines indicate the peak amplitude and steady state current, respectively. (B) Effects of TEA (20 mM). The horizontal dotted lines indicate (from top to bottom) the peak current amplitude, under control conditions and in the presence of TEA, and the steady state current, respectively. (C) Concentration dependence of inhibitory action of TEA. The Kslow conductance (G) is expressed as the fractional current using the current amplitude in TEA-free solution as unity (Gcontrol). Note that inhibition is half-maximal at 5 mM and that 30% of the current is resistant to TEA. (D) Electrical activity evoked by 15 mM glucose in the same cell before and after addition of TEA.

Mentions:
Tolbutamide, an inhibitor of KATP channels (Ashcroft and Rorsman 1989), had no effect on the current (n = 3; Fig. 4 A), but reduced the current step obtained when stepping the membrane potential from −70 to −40 mV. The latter observation indicates that some KATP channels remained active in 5 mM glucose. The Kslow current was likewise unaffected by both charybdotoxin (100 nM) and apamin (1 μM), blockers of large- and small-conductance Ca2+-activated K+ channels, respectively (not shown). By contrast, the broad spectrum K+ channel blocker tetraethylammonium (TEA)1 reduced the amplitude of the Kslow current in a concentration-dependent manner. Fig. 4 B shows an example where 20 mM TEA reduced the slowly deactivating current by ≈70%. The concentration dependence of the inhibition is summarized in Fig. 4 C. Inhibition was half-maximal at ≈5 mM TEA and ≈30% of the current was resistant to TEA. The effect of TEA on the amplitude of the current evoked by the train was associated with dramatic changes of the action potential firing pattern and the bursts of action potential were replaced by large overshooting action potentials that were either generated singly or as groups of 5–10 spikes (Fig. 4 D).

Bottom Line:
The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium.This is similar to the interval between two successive bursts of action potentials.We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

ABSTRACTWe have applied the perforated patch whole-cell technique to beta cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K(+) conductance. The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.